Investigators were searching yesterday for more “Dear John” e-mails that might provide clues to why an ex-FBI agent shot his former girlfriend and her ex-lover killing himself.

John Harrison, 53, an executive for Empire BlueCross and BlueShield, went mad with rage Monday in Midtown after his employee and former lover Isabel Munoz, 35, used office e-mail to end the affair.

“You need help. You’re sick. Leave men alone,” she said in a final, desperate electronic message to him, sources said.

Investigators got a warrant, and computer experts were searching the hard drive of Harrison’s computer for evidence of more communications.

Meanwhile, the 11th floor of the building at Broadway and 41st Street, where the murder-suicide occurred, was cordoned off as police continued their investigation.

Harrison and Munoz had been having an affair for months, and Harrison moved out of his South Jersey house to an Elizabeth, N.J., apartment two weeks ago, sources said.

But shortly before Monday’s shootings, Munoz sent Harrison the e-mail saying she wanted out.

Harrison then called Munoz and Vincent LaBianca, 33, her previous boyfriend, into his office and shot them several times before killing himself, police said.

Police said yesterday they had nothing concrete to confirm reports that Munoz had continued her involvement with LaBianca while seeing Harrison.

All three people, who worked in the company’s fraud investigation division, had survived the attack on the World Trade Center.

After the destruction of EBCBS’s downtown offices, the company moved some of its staff to the Times Square area.

Family members said the former Philadelphia cop and G-man had been traumatized by the events of 9/11.

Sources said Harrison, a veteran insurance-fraud investigator, had sought therapy after the attacks but soon stopped going.

“He got some therapy, but he didn’t think he needed any more,” Harrison’s sister said, according to a family source.

The sister had spoken to him about the affair with Munoz two weeks ago, sources said. She had been extremely concerned about Harrison’s behavior, and confirmed that the events of Sept. 11 had affected him.

Harrison had told friends he was in agony after seeing scores of dead bodies.

Neighbors in Mount Holly, where Harrison had lived with his wife for a decade, said the couple rarely socialized. “In 10 years, he must have waved a few times, but that’s it. Not the kind of guy who would going to come over and say hi.” said one neighbor.